Fix building relative URIs

Examples:

testURI --relative --base file:///a file:///b

• New correct result: b
• Old incorrect result: ../b

testURI --relative --base file:///a file:///

• New correct result: ./
• Old incorrect result: ../

testURI --relative --base file:///a/b file:///a/

• New correct result: ./
• Old incorrect result: ../../a/

This bug affects an Inkscape merge request.

This MR also adds the --relative flag to testURI and removes some unreachable code.